Method for making flat and high-density cathode for use in electrochemical cells

ABSTRACT

The traditional method of pressing CF x , screen and SVO sheet assembly results in an electrode that is cupped and not flat. This results in the reduction of the effective volumetric energy density of the electrode or the addition of a process step of flattening of the cathode if at all possible. The new method of assembly effectively eliminates the cupping behavior and produces a flat electrode. In addition, the physical density of the cathode is also increased.

BACKGROUND OF THE INVENTION

This invention relates to the conversion of chemical energy toelectrical energy. In particular, the present invention relates to anelectrode for a lithium electrochemical cell. The electrode comprises afirst cathode active material of a relatively low energy density but ofa relatively high rate capability and a second active material having arelatively high energy density but of a relatively low rate capability.The first and second active materials are short circuited to each otherby contacting the opposite sides of at least one perforated currentcollector. Alternately, the electrode can comprise spaced apart firstand second perforated current collectors, the second active materialbeing at an intermediate position with the first active materialcontacting the opposite, and outer current collector sides.

A preferred form of the cell has the electrode as a cathode connected toa terminal lead insulated from the casing serving as the negativeterminal for the anode. The present electrode design is useful forpowering an implantable medical device requiring either a medium ratepower source or a high rate discharge application. Suitable implantablemedical devices include cardiac pacemakers, cardiac defibrillators,neurostimulators, drug pumps, hearing assist devices, and the like.

In any event, the electrode needs to be relatively flat to minimize theoccupied internal volume and maximize cell energy density. The problemis that contact pressing two disparate active materials onto oppositesides of a current collector by traditional methods often provides anelectrode that is cupped. This is undesirable as it results in therebeing different impedance at the electrode periphery where spacing withthe counter anode material is relatively close in comparison to thecenter of the electrode where inter-electrode spacing is greater.Cupping also adversely reduces the cell's effective volumetric energydensity and frequently necessitates the addition of a remedial processstep for flattening the electrode, which is not always successful.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to improve theperformance of an electrochemical cell, particularly a lithiumelectrochemical cell, by improving the planarity of an electrode havingdisparate active materials contacted to opposite sides of a currentcollector. Preferably, the electrode is a cathode comprising a firstcathode active material of a relatively high rate capability, such asSVO, contacted to one side of the current collector with a secondcathode active material of a relatively high energy density, such asCF_(x), contacted to the other side. In that manner, the separate SVOand CF_(x) materials are short-circuited to each other through theperforated current collector. Providing the active materials in a shortcircuit relationship means that their respective attributes of high rateand high energy density benefit overall cell discharge performance.

These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description and to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a cathode 10 according to one embodiment of theinvention having first and second active materials 14, 16 contacted toopposite sides of a perforated current collector 12.

FIG. 2 is a schematic of a cathode 20 comprising materials similar tothose used to construct the cathode of FIG. 1, but having been made by aprior art pressing process.

FIG. 3 is a schematic embodiment of a cathode 30 according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrochemical cell that possesses sufficient energy density anddischarge capacity required to meet the vigorous requirements ofimplantable medical devices comprises an anode of a metal selected fromGroups IA and IIA of the Periodic Table of the Elements. Such anodeactive materials include lithium, sodium, potassium, etc., and theiralloys and intermetallic compounds including, for example, Li—Si, Li—Al,Li—B and Li—Si—B alloys and intermetallic compounds. The preferred anodecomprises lithium. An alternate anode comprises a lithium alloy such asa lithium-aluminum alloy. The greater the amounts of aluminum present byweight in the alloy, however, the lower the energy density of the cell.

The form of the anode may vary, but preferably the anode is a thin metalsheet or foil of the anode metal, pressed or rolled on a metallic anodecurrent collector, i.e., preferably comprising titanium, titanium alloyor nickel, to form an anode component. Copper, tungsten and tantalum arealso suitable materials for the anode current collector. In theexemplary cell of the present invention, the anode current collector hasan extended tab or lead contacted by a weld to a cell case of conductivemetal in a case-negative electrical configuration.

The electrochemical cell further comprises a cathode of electricallyconductive material that serves as the counter electrode. The cathode isof solid materials and the electrochemical reaction at the cathodeinvolves conversion of ions that migrate from the anode to the cathodeinto atomic or molecular forms. The solid cathode may comprise a firstactive material of a metal element, a metal oxide, a mixed metal oxideand a metal sulfide, and combinations thereof, and a second activematerial of a carbonaceous chemistry. The metal oxide, the mixed metaloxide and the metal sulfide of the first active material has arelatively lower energy density but a relatively higher rate capabilitythan the second active material.

The first active material is formed by the chemical addition, reaction,or otherwise intimate contact of various metal oxides, metal sulfidesand/or metal elements, preferably during thermal treatment, sol-gelformation, chemical vapor deposition or hydrothermal synthesis in mixedstates. The active materials thereby produced contain metals, oxides andsulfides of Groups, IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII, whichincludes the noble metals and/or other oxide and sulfide compounds. Apreferred cathode active material is a reaction product of at leastsilver and vanadium.

One preferred mixed metal oxide is a transition metal oxide having thegeneral formula SM_(x)V₂O_(y) where SM is a metal selected from GroupsIB to VIIB and VIII of the Periodic Table of Elements, wherein x isabout 0.30 to 2.0 and y is about 4.5 to 6.0 in the general formula. Byway of illustration, and in no way intended to be limiting, oneexemplary cathode active material comprises silver vanadium oxide havingthe general formula Ag_(x)V₂O_(y) in any one of its many phases, i.e.,β-phase silver vanadium oxide having in the general formula x=0.35 andy=5.8, γ-phase silver vanadium oxide having in the general formulax=0.74 and y=5.37 and ε-phase silver vanadium oxide having in thegeneral formula x=1.0 and y=5.5, and combinations and mixtures of phasesthereof. For a more detailed description of such cathode activematerials reference is made to U.S. Pat. No. 4,310,609 to Liang et al.,which is assigned to the assignee of the present invention andincorporated herein by reference.

Another preferred composite transition metal oxide cathode materialincludes V₂O_(z) wherein z≦5 combined with Ag₂O with silver in eitherthe silver(II), silver(I) or silver(0) oxidation state and CuO withcopper in either the copper(II), copper(I) or copper(0) oxidation stateto provide the mixed metal oxide having the general formulaCu_(x)Ag_(y)V₂O_(z), (CSVO). Thus, the composite cathode active materialmay be described as a metal oxide-metal oxide-metal oxide, a metal-metaloxide-metal oxide, or a metal-metal-metal oxide and the range ofmaterial compositions found for Cu_(x)Ag_(y)V₂O_(z) is preferably about0.01≦z≦6.5. Typical forms of CSVO are Cu_(0.16)Ag_(0.67)V₂O_(z) with zbeing about 5.5 and Cu_(0.16)Ag_(0.67)V₂O_(z) with z being about 5.75.The oxygen content is designated by z since the exact stoichiometricproportion of oxygen in CSVO can vary depending on whether the cathodematerial is prepared in an oxidizing atmosphere such as air or oxygen,or in an inert atmosphere such as argon, nitrogen and helium. For a moredetailed description of this cathode active material reference is madeto U.S. Pat. Nos. 5,472,810 to Takeuchi et al. and 5,516,340 to Takeuchiet al., both of which are assigned to the assignee of the presentinvention and incorporated herein by reference.

The cathode design of the present invention further includes a secondactive material of a relatively high energy density and a relatively lowrate capability in comparison to the first cathode active material. Thesecond active material is preferably a carbonaceous compound preparedfrom carbon and fluorine, which includes graphitic and non-graphiticforms of carbon, such as coke, charcoal or activated carbon. Fluorinatedcarbon is represented by the formula (CF_(x))_(n) wherein x variesbetween about 0.1 to 1.9 and preferably between about 0.2 and 1.2, and(C₂F)_(n) wherein the n refers to the number of monomer units, which canvary widely.

In particular, it is generally recognized that for lithium cells, silvervanadium oxide (SVO), and specifically E-phase silver vanadium oxide(AgV₂O_(5.5)), is preferred as the cathode active material. This activematerial has a theoretical volumetric capacity of 1.37 Ah/ml. Bycomparison, the theoretical volumetric capacity of CF_(x) material(x=1.1) is 2.42 Ah/ml, which is 1.77 times that of E-phase silvervanadium oxide. For powering a cardiac defibrillator, SVO is preferredbecause it can deliver high current pulses or high energy within a shortperiod of time. Although CF_(x) has higher volumetric capacity, itcannot be used in medical devices requiring a high rate dischargeapplication due to its low to medium rate of discharge capability.

In a broader sense, it is contemplated by the scope of the presentinvention that the first cathode active material is any material thathas a relatively lower energy density but a relatively higher ratecapability than the second active material. In addition to silvervanadium oxide and copper silver vanadium oxide, V₂O₅, MnO₂, LiCoO₂,LiNiO₂, LiMn₂O₄, TiS₂, Cu₂S, FeS, FeS₂, copper oxide, copper vanadiumoxide, and mixtures thereof are useful as the first active material.And, in addition to fluorinated carbon, Ag₂O, Ag₂O₂, CuF, Ag₂CrO₄, MnO₂,and even SVO itself, are useful as the second active material. Thetheoretical volumetric capacity (Ah/ml) of CF_(x) is 2.42, Ag₂O₂ is3.24, Ag₂O is 1.65 and AgV₂O_(5.5) is 1.37. Thus, CF_(x), Ag₂O₂, Ag₂O,all have higher theoretical volumetric capacities than that of SVO.

Before fabrication into an electrode structure for incorporation into anelectrochemical cell according to the present invention, the firstcathode active material prepared as described above is preferably mixedwith a binder material such as a powdered fluoro-polymer, morepreferably powdered polytetrafluoroethylene or powdered polyvinylideneflouride present at about 1 to about 5 weight percent of the cathodemixture. Further, up to about 10 weight percent of a conductive diluentis preferably added to the first cathode mixture to improveconductivity. Suitable materials for this purpose include acetyleneblack, carbon black and/or graphite or a metallic powder such aspowdered nickel, aluminum, titanium, and stainless steel. The preferredfirst cathode active mixture thus includes a powdered fluoro-polymerbinder present at about 3 weight percent, a conductive diluent presentat about 3 weight percent and about 94 weight percent of the cathodeactive material. The second cathode active mixture includes afluoro-polymer binder present at about 1 to 4 weight percent, aconductive diluent present at about 1 to 10 weight percent and about 86to 98 weight percent of the cathode active material. A preferred secondactive mixture is, by weight, 91% to 98% CF_(x), 4% to 1% PTFE and 5% to1% carbon black. A novel electrode construction using both a high rateactive material, such as SVO, and a high energy density material, suchas CF_(x), is described in U.S. Pat. No. 6,551,747 to Gan. This patentis assigned to the assignee of the present invention and incorporatedherein by reference.

Cathodes for incorporation into an electrochemical cell may be preparedby rolling or pressing a quantity of the active material mixtures toform a blank prior to contact with a current collector. Alternatively,the active mixture may be provided in the form of a free-standing sheet.This is done by first adjusting the particle size of the cathode activematerial to a useful size followed by mixing with binder and conductiveadditives suspended in a suitable solvent to form a paste. The paste isthen fed into a series of roll mills to form the sheet material, or thepaste can first be pelletized before the rolling step. The cathode sheetmaterial is dried and punched into cathode plates or blanks of thedesired shape. For a more detailed description of the preparation offree-standing active sheets, reference is made to U.S. Pat. No.5,435,874 to Takeuchi et al., which is assigned to the assignee of thepresent invention and incorporated herein by reference.

Regardless, blanks of the first and second cathode active materials aresupported on a suitable current collector selected from the groupconsisting of stainless steel, titanium, tantalum, platinum, aluminum,gold, nickel, and alloys thereof. The preferred current collectormaterial is titanium, and most preferably the titanium cathode currentcollector has a thin layer of graphite/carbon paint applied thereto. Thecurrent collector has a thickness from about 0.001 inches to about 0.01inches, about 0.002 inches thick being preferred. For a more detaileddescription of a carbonaceous coating on a titanium current collector,reference is made to U.S. Pat. No. 6,767,670 to Paulot et al., which isassigned to the assignee of the present invention and incorporatedherein by reference.

The conventional method of contacting disparate active materials onopposite sides of a perforated current collector is done by firstpressing a CF_(x) blank at a first pressure. An SVO sheet or blank inthen placed in the bottom of a pressing fixture followed by the currentcollector and finally the CF_(x) blank. If desired, the SVO blank andCF_(x) blank can be loaded into the fixture in an opposite order. In anyevent, this assembly is pressed together at a second pressure higherthan the first pressure used to form the CF_(x) blank. The problem isthat this technique often results in cupping of the electrode with theCF_(x) blank curving toward the SVO. This cupping can be by 30%, orgreater.

The present method involves first pressing the CF_(x) blank at a firstpressure. Next, an already manufactured SVO blank or sheet is placed inthe bottom of a pressing fixture followed by the current collector andfinally the CF_(x) blank. As before, the SVO and CF_(x) blanks can beloaded into the fixture in an opposite order. This assembly is thenpressed together at a second pressure equal to or less than the firstpressure used to form the CF_(x) blank. The electrode assembly has theconfiguration: first active material/current collector/second activematerial, or in the preferred embodiment SVO/current collector/CF_(x),with cupping in a range of about 0% to about 5%, preferably about 2%,and below.

Suitable pressing pressures range from about 0.1 tons/cm² to about 10tons/cm², and more preferably about 0.2 tons/cm² to about 6 tons/cm².Dwell time at maximum pressure is about 1 second to about 1 minute, morepreferably about 2 seconds to 30 seconds.

In addition to improving the cupping characteristics of the electrode,the density of the CF_(x) material is increased by about 6% to 15% overthat obtainable by conventional manufacturing methods, which benefitsthe cell's energy density. This not only results in a higher effectivemechanical density, thereby reducing the size of the electrochemicalcell, but it also simplifies cell construction. A cupped electrode thatneeds to be mechanically flattened in a remedial step not only increasesthe risk of damage to the electrode, but also adds an additional processstep.

The mechanism for the improvement is as follows. The CF_(x) cathodeactive material contains carbon as one of its ingredients. When theCF_(x) blank is pressed at a relatively low pressure, it undergoes acertain amount of compression and upon removing the pressure, somerelaxation. Next, during the assembly process when the SVO blank,current collector and CF_(x) blank are pressed at a relatively higherpressure, the disparate active materials undergo additional compression.Each of the CF_(x) and SVO blanks comprises first and second major sidesseparated by a peripheral sidewall. The perforated current collector“captures” the major side of the CF_(x) blank that it contacts,preventing the blank from expanding (or relaxing) there. However, theother major side of the CF_(x) blank is free to expand after thecompression pressure is removed, and when it does so, the electrodedeflects or cups towards the current collector and SVO blank.

In the present pressing process, however, this is prevented fromhappening. When the CF_(x) blank is pressed at a first pressure, itattains a relatively higher density. During the electrode assemblyprocess, the compression pressure is the same as or less than the firstCF_(x) blanking pressure. Since the CF_(x) portion of the electrodeassembly is already at a relatively higher density, the second pressingdoes not significantly increase the density of the CF_(x) material.Consequently, the degree of relaxation experienced by the CF_(x)material is relatively small. Also, the first and second major sides ofthe CF_(x) blank relax at relatively similar rates. This preventscupping while obtaining a high physical density.

FIG. 1 is a schematic view of one embodiment of a cathode electrode 10according to the present invention. Electrode 10 comprises a perforatedcurrent collector 12 having opposed major sides 12A and 12B. A firstcathode active material 14 of a relatively high energy density but arelatively low rate capability, preferably CF_(x), is contacted to thefirst major current collector side 12A. A second cathode active materialof a relatively high rate capability but a relatively low energydensity, preferably SVO, is contacted to the other major currentcollector side 12B.

The first cathode active material 14 has first and second major sides14A, 14B, the latter being in direct contact with the first currentcollector side 12A. Similarly, the second cathode active material 16 hasfirst and second major sides 16A, 16B, the former being in directcontact with the second current collector side 12B. In an idealconstruction, there is no cupping of the electrode prior toincorporation into an electrochemical cell. This means that the majorsides 14A, 14B of the first active material 14 are parallel to eachother as well as are the major sides 16A, 16B of the second major activematerial. Consequently, the first and second major sides 12A, 12B of thecurrent collector 12 are also parallel to each other and to therespective major sides of the active materials 14, 16. As previouslydescribed, at the most the major sides of the various electrodecomponents deflect or cup by about 5%, or less.

FIG. 2 illustrates a schematic representation of a prior art electrode20 exhibiting significant cupping. In this drawing, the currentcollector 22 has major sides 22A, 22B, the first active material 24 hasmajor sides 24A, 24B, the latter contacting side 22A of the currentcollector, and the second active material 26 has major sides 26A, 26B,the former being in contact with side 22B of the current collector.Because the first active blank 24 was initially pressed at a firstpressure followed by the electrode assembly being pressed at a second,greater pressure, the first major side 24A of the first active materialrelaxes at a greater rate than the second major side 24B “captured” bythe first side 22A of the current collector. This relaxation force issufficient to deflect the entire electrode assembly toward the secondelectrode blank 26. The degree of cupping is shown by the distance “x”,which is measured at the center point 26C of the second blank withrespect to a vertical imaginary line 26D passing through the spacedapart ends 26E, 26F of the blank. A distance “y” is measured from apoint where lines x and 26D intersect to either ones of the edges 26E,26F.

In the prior art construction an imaginary tangent line 26G passingthrough the center point 26C is parallel to imaginary line 26D, butspaced there from. In an ideal construction according to the presentinvention, the lines 26D and 26G are co-linear. The degree of deflectionis calculated by dividing the distance x by the distance y times 100.

FIG. 3 is a schematic view of a portion of a cathode electrode 30according to another embodiment of the present invention. This electrode30 is preferably built by loading into the pressing fixture the variousparts needed to make the electrode assembly 10 shown in FIG. 1 alongwith a second perforated current collector 32 having its major side 32Bcontacting the major side 14A of blank 14. A third blank 34 of a thirdactive material, which is preferably SVO, is placed in the pressingfixture in contact with the bare side 32A of current collector 32 andthe entire assembly is then pressed together. As before, the pressingpressure is equal to or less than that which was originally used to formblank 14. While the second and third blanks 16, 34 are described aspreferably comprising SVO that is not necessary. They can be the same ordifferent materials as long as each of them is of a relatively greaterrate capability but a lesser energy density than the material of thefirst blank 14. Blank 14 is now sandwiched between and in direct, shortcircuit contact with the inner major sides 12A and 34B of the respectivecurrent collectors 12, 34

In that respect, cathodes prepared as described above may be in the formof one or more plates operatively associated with at least one or moreplates of anode material, or in the form of a strip wound with acorresponding strip of anode material in a structure similar to a“jellyroll”. While not shown in FIGS. 1 and 3, the cathode currentcollectors 32 and 34 are connected to a common terminal insulated fromthe cell casing (not shown) by a suitable glass-to-metal seal. Thisdescribes a case-negative cell design, which is the preferred form ofthe present cell. The cell can also be built in a case-positive designwith the cathode current collectors contacted to the casing and theanode current collector connected to a terminal lead insulated from thecasing. In a further embodiment, the cell is built in a case-neutralconfiguration with both the anode and the cathode connected torespective terminal leads insulated from the casing. These terminalconstructions are well known by those skilled in the art.

In order to prevent internal short circuit conditions, the sandwichcathode is separated from the Group IA, IIA or IIIB anode by a suitableseparator material. The separator is of electrically insulativematerial, and the separator material also is chemically unreactive withthe anode and cathode active materials and both chemically unreactivewith and insoluble in the electrolyte. In addition, the separatormaterial has a degree of porosity sufficient to allow flow there throughof the electrolyte during the electrochemical reaction of the cell.Illustrative separator materials include fabrics woven fromfluoropolymeric fibers including polyvinylidine fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethyleneused either alone or laminated with a fluoropolymeric microporous film,non-woven glass, polypropylene, polyethylene, glass fiber materials,ceramics, polytetrafluoroethylene membrane commercially available underthe designation ZITEX (Chemplast Inc.), polypropylene/polyethylenemembrane commercially available under the designation CELGARD (CelanesePlastic Company, Inc.), a membrane commercially available under thedesignation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.), and apolyethylene membrane commercially available from Tonen Chemical Corp.

The electrochemical cell of the present invention further includes anonaqueous, ionically conductive electrolyte that serves as a medium formigration of ions between the anode and the cathode electrodes duringthe electrochemical reactions of the cell. The electrochemical reactionat the electrodes involves conversion of ions in atomic or molecularforms that migrate from the anode to the cathode. Thus, nonaqueouselectrolytes suitable for the present invention are substantially inertto the anode and cathode materials, and they exhibit those physicalproperties necessary for ionic transport, namely, low viscosity, lowsurface tension and wettability.

A suitable electrolyte has an inorganic, ionically conductive saltdissolved in a mixture of aprotic organic solvents comprising a lowviscosity solvent and a high permittivity solvent. In the case of ananode comprising lithium, preferred lithium salts that are useful as avehicle for transport of alkali metal ions from the anode to the cathodeinclude LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiO₂, LiAlCl₄, LiGaCl₄,LiC(SO₂CF₃)₃, LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₃, LiC₆F₅SO₃, LiO₂CCF₃,LiSO₆F, LiB(C₆H₅)₄, LiCF₃SO₃, and mixtures thereof.

Low viscosity solvents useful with the present invention include esters,linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran(THF), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethylcarbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE),1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propylcarbonate, ethyl propyl carbonate, diethyl carbonate, dipropylcarbonate, and mixtures thereof, and high permittivity solvents includecyclic carbonates, cyclic esters and cyclic amides such as propylenecarbonate (PC), ethylene carbonate (EC), butylene carbonate,acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethylacetamide, γ-valerolactone, γ-butyrolactone (GBL),N-methyl-2-pyrrolidone (NMP), and mixtures thereof. In the presentinvention, the preferred anode is lithium metal and the preferredelectrolyte is 0.8M to 1.5M LiAsF₆ or LiPF₆ dissolved in a 50:50mixture, by volume, of propylene carbonate and 1,2-dimethoxyethane.

The corrosion resistant glass used in the glass-to-metal seals has up toabout 50% by weight silicon such as CABAL 12, TA 23, FUSITE 425 orFUSITE 435. The positive terminal leads preferably comprise molybdenum,although titanium, aluminum, nickel alloy, or stainless steel can alsobe used. The cell casing is an open container of a conductive materialselected from nickel, aluminum, stainless steel, mild steel, tantalumand titanium. The casing is hermetically sealed with a lid, typically ofa material similar to that of the casing.

Benefits attributed to the present process are illustrated by thefollowing examples:

EXAMPLES

In the traditional method, the CF_(x) material is blanked at a pressureof about 0.24 tons/cm² for about 20 seconds. The resulting CF_(x) blankis then contacted to one side of a perforated current collector havingan SVO blank contacted to the other side thereof. This assembly is thenpressed together at about 4 tons/cm² for about 10 seconds. The resultingcathode exhibits cupping of about 30% to about 60%.

According to the present method, the CF_(x) material is blanked at 4.56tons/cm² for about 10 seconds. The resulting CF_(x) blank is thencontacted to one side of a perforated current collector having an SVOblank contacted to the other side thereof. This assembly is thensubjected to a pressure of about 3.61 tons/cm² for about 10 seconds. Thepresent process resulted in cathodes that were less than 2% cupped.Additionally, the total process time has been reduced by 10 seconds froma total of 30 seconds for the conventional method to a total of about 20seconds for the present process.

It is appreciated that various modifications to the inventive conceptsdescribed herein may be apparent to those of ordinary skill in the artwithout departing from the present invention as defined by the appendedclaims.

1. A method for providing an electrochemical cell, comprising the stepsof: a) providing an anode; b) providing a cathode comprising the stepsof: i) pressing a first cathode active material having a relatively highenergy density but a relatively low rate capability at a first pressureto form a first cathode active material blank; ii) positioning the firstcathode active blank in a pressing fixture contacting a first major sideof a first cathode current collector and a second cathode active blankof a material having a relatively low energy density but a relativelyhigh rate capability contacting a second, opposite major side of thefirst cathode current collector to form a cathode assembly; and iii)pressing the cathode assembly at a second pressure that is less than thefirst pressure; c) providing a separator intermediate the anode andcathode in electrical association with each other; and d) activating theanode and cathode with a electrolyte.
 2. The method of claim 1 includingproviding the first pressure at about 0.1 tons/cm² to about 10 tons/cm².3. The method of claim 1 including performing the first and secondpressings for a dwell time of about 2 seconds to about 60 seconds atmaximum pressure.
 4. The method of claim 1 including providing thesecond cathode active material having first and second sides, the firstside contacting the cathode current collector and the second sidecupping by about 5%, or less.
 5. The method of claim 1 includingselecting the first cathode active material from CF_(x) and C₂F.
 6. Themethod of claim 1 including selecting the second cathode active materialfrom the group consisting of SVO, CSVO, V₂O₅, MnO₂, LiCoO₂, LiNiO₂,LiMnO₂, CuO₂, TiS₂, Cu₂S, FeS, FeS₂, copper vanadium oxide and mixturesthereof.
 7. The method of claim 1 wherein the anode is lithium, thefirst cathode active material is CF_(x), and the second cathode activematerial is SVO.
 8. The method of claim 1 including positioning a secondcathode current collector in the pressing fixture with its first majorside contacting the first cathode active blank opposite the firstcathode current collector and a third cathode active material blankhaving first and second sides, the first side contacting the secondmajor side of the second cathode current collector and further includingpressing this assembly at a third pressure no greater than the firstpressure with the second side of the third cathode active blank notcontacting the second current collector cupping by about 5%, or less. 9.The method of claim 8 including providing the first and second currentcollectors being of titanium having a layer of graphite/carbon contactedthereto.
 10. The method of claim 1 including providing the currentcollector having a thickness from about 0.001 inches to about 0.01inches.
 11. The method of claim 1 including providing the anode oflithium in the form of at least one plate comprising an anode currentcollector electrically connected to a casing as its terminal and thecathode connected to a cathode terminal insulated from the casing.
 12. Amethod for providing an implantable medical device, comprising the stepsof: a) providing the implantable medical device except for its powersource; b) providing a power source comprising an anode and a cathodehoused inside a casing and segregated from direct contact with eachother by an intermediate separator, the anode and cathode beingactivated by an electrolyte, providing the cathode comprising the stepsof: i) pressing a first cathode active material having a relatively highenergy density but a relatively low rate capability at a first pressureto for a first cathode active material blank; ii) positioning the firstcathode active blank in a pressing fixture contacting a first major sideof a first cathode current collector and a second cathode active blankof a material having a relatively low energy density but a relativelyhigh rate capability contacting a second, opposite major side of thefirst cathode current collector to form a cathode assembly; and iii)pressing the cathode assembly at a second pressure that is less than thefirst pressure; and c) electrically connecting the power source to themedical device to provide a functional implantable medical device. 13.The method of claim 12 including selecting the implantable medicaldevice from the group consisting of cardiac pacemakers, cardiacdefibrillators, neurostimulators, drug pumps, and hearing assistdevices.
 14. A method for providing an electrochemical cell, comprisingthe steps of: a) providing an anode; b) providing a cathode comprisingthe steps of: i) pressing a first cathode active material selected fromCF_(x) and C₂F at a first pressure to form a first cathode activematerial blank; ii) positioning the first cathode active blank in apressing fixture contacting a first major side of a first cathodecurrent collector and a second cathode active blank of a materialselected from the group consisting of SVO, CSVO, V₂O₅, MnO₂, LiCoO₂,LiNiO₂, LiMnO₂, CuO₂, TiS₂, Cu₂S, FeS, FeS₂, copper vanadium oxide andmixtures thereof contacting a second, opposite major side of the firstcathode current collector to form a cathode assembly; and iii) pressingthe cathode assembly at a second pressure that is less than the firstpressure; c) providing a separator intermediate the anode and cathode inelectrical association with each other; and d) activating the anode andcathode with a electrolyte.
 15. The method of claim 14 includingproviding the first pressure at about 0.1 tons/cm² to about 10 tons/cm².16. The method of claim 14 including providing the first pressure atabout 0.2 tons/cm² to about 6 tons/cm².
 17. The method of claim 14including performing the first and second pressings for a dwell time ofabout 2 seconds to about 60 seconds at maximum pressure.
 18. The methodof claim 14 including providing the second cathode active materialhaving first and second sides, the first side contacting the cathodecurrent collector and the second side cupping by about 5%, or less. 19.The method of claim 14 including pressing the first cathode activematerial at the first pressure being about 4.56 tons/cm² and thenpressing the cathode assembly at the second pressure being about 3.61tons/cm².